1. Field of the Invention
The present invention relates to a method for producing killed steels, such as Al-killed, Si-killed and Al-Si-killed steels for continuous casting.
2. Description of the Prior Art
Conventional methods for producing Al-killed, Si-killed or Al-Si-killed steels comprise controlling the oxygen blowing to a converter so as to obtain a steel composition and temperature predetermined for a specific steel grade, while adding alloying elements to the converter, adjusting the steel composition by adding alloying elements on the basis of sampling results at the finishing stage or the blow-off stage of the oxygen blowing or at the time of tapping, and supplying the molten steel thus obtained to a continuous casting machine through a ladle and a tundish. Therefore, in the conventional methods, the converter is subjected to severe operation conditions for a long period of time, and the operation conditions vary depending on grades of steel to be produced, so that the control of the oxygen-blown refining in the converter is very complicated.
For example, for producing of a low-carbon Al-killed steel containing 0.09% or less carbon for continuous casting, the carbon content at the time of blow termination (blow-off carbon content) is maintained in a range from 0.03 to 0.06% in view of the increase in carbon content from addition of Fe-Mn, etc. to the ladle during the pouring so that the total Fe % in the slag exceeds 20%, thus producing excessive oxidized molten steel which causes considerable shortening of the converter and ladle refractory linings, as well as loss of iron yield in the molten steel. The above disadvantages caused by the excessively oxidized molten steel have been regarded as being unavoidable and inherent to the conventional methods, and resulted in considerable fluctuation in the blown-off temperature and the steel composition in the converter operation according to the conventional methods.
Further, due to the excessively oxidized condition of the molten steel, the manganese content at the blow-off is 0.13% or less when the blow-off carbon content is from 0.03 to 0.06%. Therefore, in order to obtain a predetermined steel composition, the addition of a larger amount of Fe-Mn (for example 3 kg/ton of molten steel) is required, and for this addition, a low-carbon Fe-Mn is required because the carbon content in the final product very often exceeds its upper limit due to pick-up of carbon from Fe-Mn and causes rejects. However, the low-carbon Fe-Mn, as compared with a high-carbon Fe-Mn, requires a much large power consumption for its production and costs about two times more than the high-carbon Fe-Mn. Therefore, the use of a low-carbon Fe-Mn will cause disadvantages in the production cost of molten steel.
Further, the excessive oxidation of molten steel lowers the yield rate of ferro-alloys and increases fluctuation in the steel compositions so that in the conventional methods one is required to predetermine the target values for the final product at a considerably higher level than the actual values and to provide a wider range of tolerance. Thus, the above disadvantages due to the excessive oxidation of the molten steel have been unavoidable and inherent in the production of low-carbon Al-killed or Si-killed molten steels for continuous casting because of the necessity of maintaining the blow-off carbon content in a range from 0.03 to 0.06%.
Further, according to the conventional methods, Al or Si is added to the molten steel during pouring after the blow-termination or to the ladle after the pouring, so that in the case of the ordinary addition during the pouring, only less than 25% Al addition yield and about 40 to 80% Si addition yield can be achieved. In the case of a special and complicated addition, such as, high-speed addition in the forms of Al wire or Al bullet, and addition under non-oxidizing atmosphere and/or under stirring, only about 30 to 40% addition yield can be achieved for Al and only about 50 to 80% addition yield can be achieved for Si. Thus in the conventional methods, the loss of Al and Si during their addition to the molten steel is very large. In the case of Si addition, even when Al is added in an amount from 0.001 to 0.008% as the total Al so as to stabilize the Si addition, only 60 to 85% addition yield with considerable fluctuation can be achieved. In this way, a considerable amount of alumina and/or oxide inclusions are produced in the molten steel due to the loss of Al and/or Si during their addition, and these inclusions cause not only deterioration of the molten steel quality, but also difficulties in the continuous casting operation, such as, clogging of the pouring nozzle.
Further, according to the conventional methods, addition of elements other than Al and Si has been effected simultaneously with Fe-Mn-Al or Fe-Mn-Si to the molten steel during the pouring or to the ladle after the pouring. In this case, also, just as Fe-Mn, Al or Si, the addition yield of other elements is low and fluctuates considerably due to the excessive oxidation of the molten steel.
As another disadvantage of the conventional methods, it is required that the pouring temperature of the molten steel from the converter is set so as to assure a molten steel temperature within the tundish 20.degree. to 40.degree. C. higher than the solidification temperature. Thus, the molten steel temperature within the tundish is not higher than the solidification point by more than 20.degree. C., a large amount of alumina or oxide adhesion is formed around the pouring nozzle, and this causes early clogging of the nozzle and hence difficulties in continuing a smooth casting operation.
On the other hand, if the molten steel temperature within the tundish is higher than the solidification by more than 40.degree. C., the solidification speed in the mold is lowered, and this causes slab surface defects, such as, slag or powder entrappment. In order to prevent such surface defects, the casting speed must be limited to an appropriate speed. For this reason, in the case of Al-killed steels, 10 to 30% surface conditioning is required as shown in FIG. 8, and in the case of Si-killed steels about 15% surface conditioning is required.
Further, even when the temperature within the tundish is maintained 20.degree. to 40.degree. C. higher than the solidifying temperature and a "bank" is provided within the tundish or the shape of the immersion nozzle is improved so as to remove the alumina or oxide inclusions, a completely satisfactory result can not be achieved and as shown in FIG. 6, the alumina cluster or oxide inclusions segregate in the thickness direction of the slab, and when such slabs are used for production of cold rolled steel sheets, the surfacial portion of the slab is conditioned and removed after the coating. This causes considerable lowering of the iron yield of the slabs. In the case of a continuous casting machine of curved strand type, the alumina clusters or oxide inclusions which segregate at 1/4 thickness portions of the slabs, are exposed as sliver defects on the surfaces of the cold rolled steel sheets produced from such slabs, thus causing considerable lowering of the product yield as shown in FIG. 7.
In FIG. 6, the solid line (1) represents the distribution of alumina clusters in a slab produced by adding the total amount of Fe-Mn and Al during the pouring, while the chained line (2) represents the distribution of alumina cluster in a slab produced by adding only Fe-Mn during the pouring and adding Al under a non-oxidizing atmosphere after the pouring with stirring.
As mentioned above, Al-killed, Si-killed or Al-Si-killed steel products having satisfactory inner and surfacial qualities could not be obtained by the conventional methods.
Meanwhile, in order to eliminate the defects of Al-killed, Si-killed or Al-Si-killed steels produced by the conventional methods, various experiments have been tried, including pouring the molten steel under non-oxidized or semi-oxidized conditions to a ladle and subjecting the molten steel to a vacuum degassing treatment.
However, the degassing operation has been regarded and established as production means only for production of extremely low-hydrogen and extremely low-carbon steels for high-grade thick plates, and the degassing treatment has been performed under the conditions of 1-5 mmHg vacuum rate and 4-10 circulations (definitions will be set forth hereinafter), so that a large scale of a vacuum generator as well as a long period of treating time has been required, resulting in a considerable temperature lowering during the treatment. Therefore, it is necessary to maintain the blow-off temperature in the converter 20.degree. to 50.degree. C. higher as compared with an ordinary non-degassed molten steel in order to maintain the molten steel temperature within the tundish 20.degree. to 40.degree. C. higher than the solidifying temperature as mentioned before. This procedure, however, causes remarkable loss and damage of the refractories of the converter and the ladle, as well as the degassing equipment, and increases the consumption of various energy sources, such as, vapour for the vacuum generator, and power and Ar gas for the degassing equipment, resulting in increased total cost of the degassing treatment. Thus, vacuum degassing of ordinary molten steels, such as, Al-killed, Si-killed or Al-Si-killed steels for continuous casting would result in severe damage of the refractories of the converter, the ladle and the degassing equipment, and a remarkable increase in the degassing cost.
Further, in the conventional methods, as most parts of Mn, Si and Al for composition adjustment are added during the pouring, their addition yield is low while the N content in the steel increases considerably. This is not desirable for the production of steel grades which require a low N content. In addition, during the addition of Mn, Si and Al, the hydrogen (H) content in the steel increases due to the water adhering to these additives, so that it is necessary to remove this increased hydrogen content and for this removal, the vacuum degassing is performed under a high degree of operation load.
In order to improve the addition yield of the above elements, and to lower the hydrogen (H) and (N) contents in the steel, it has been proposed to pour a molten steel prepared in a converter to a ladle under a non-deoxidized or semi-oxidized condition, and to add the above elements during a high-vacuum degassing treatment.
Meanwhile, when non-deoxidized molten steel is subjected to vacuum degassing under conventional conditions, the splash phenomenon, which is caused by a decarburization reaction during the treatment is very remarkable, and particularly in the case of molten steels containing 0.05% or more blow-off carbon content, so that a smooth degassing treatment can not be easily achieved and equipmental troubles result such that the molten steel blows from the degassing vessel into the vacuum exhausting system. Therefore, up to now, the vacuum degassing treatment has not been applied to Al-killed, Si-killed or Al-Si-killed steels for continuous casting because of these great disadvantages.